Where is bore size measured?

[Tubajug]

Is it the lead pipe? Valve set? Just curious...

[Rick F]

I believe it's inside the diameter of 2nd valve tuning slide.

If you have a compensator, that bore meas. it taken inside of the corresponding tuning slide.

[SousaSaver]

I believe it's inside the diameter of 2nd valve tuning slide.

If you have a compensator, that bore meas. it taken inside of the corresponding tuning slide.

BINGO!

Also, if you have a horn with a graduated bore, the 4th valve may have a larger bore than the first three.

[sailn2ba]

WOW. The most informative posts are the shortest threads.
Not to say that I don't enjoy discussions!

[sloan]

WOW. The most informative posts are the shortest threads.
Not to say that I don't enjoy discussions!

So...let's make it longer: why is the bore for rotaries so large, compared with pistons?

[TubaBobH]

WOW. The most informative posts are the shortest threads.
Not to say that I don't enjoy discussions!

So...let's make it longer: why is the bore for rotaries so large, compared with pistons?

I have always thought that the reason the bore for rotaries is large compared to pistons is due to the placement of the valve packs. Piston valves, tend to be much closer to the lead pipe than rotary valves. Therefore, the bore of the second valve tuning slide for piston tubas would tend to be smaller than the bore of the second valve tuning slide for rotary tubas.

[Donn]

Piston valves, tend to be much closer to the lead pipe than rotary valves.

Though of course this doesn't establish a causal relationship - are they smaller because they're closer to the small end, or are they closer to the small end because they're smaller?

To the rotor's advantage, the relationship between rotor travel and finger travel is a matter of a choosing a compromise between travel and mechanical advantage. Where the best the piston can do is squash the ports (e.g., Conn 20K.)

[Alex C]

I think it has more to do with American vs German design - like the classic small bore York-ish compared to the Kaiser large bore. Although there are some models of piston valve tubas and sousaphones with bores above rotary valves (like old Conns and Kings).

Larger bore pistons seem to be making their way back, Miraphone IIRC has a couple with .835" or somewhere in there.

The classic York tubas were not especially small bore at .750". The 4th valve tubing was larger on those horns, but the ports on the 4th valve were the same size as the other three valves. Go figure.

The introduction of pistons with a large bore is new. The Conn 24J had the largest pistons and bore I have seen until recently. Conn 'squished' the ports to shorten the length on that piston and it was only a .775" bore.

The humongous Martins had a .689" bore and I think the pistons were dainty compared with the York/Holton valve sets.

Giant pistons are harder to build because the ports of the piston must be balled out to precise size, where as a rotor is only a half round and is cut by a machine.

The ports in pistons are inserts, not cut through a solid piece of brass. Sadly, they are not balled out or precise.

Dan Oberloh has a wonderful picture article on making pistons, I think he built pistons for a 4 Holtons at one time for this article. Here: http://www.oberloh.com/gallery/tubavalvereferbish.htm"

I believe that the problem with large size pistons is a manufacturing issue that had to do with the amount of surface contact between piston and casing. Working with a factory spec of .005” clearence it took really excellent machining to get to factory specs (much less design specs). Computer driven milling machines have improved the chances for success.

Incidentally, rotary valves used to be made by assembling pieces as well pistons. The rotor consisted of 1 center post, at least 2 pieces for a top and a bottom, 2 ports and 1 piece of brass used to make the outside of the rotor. No large pieces were milled. Rotors are all milled from one piece of brass now.

[windshieldbug]

The deal is, when horns of ANY kind have a standard shape and length you are pretty accurate when you measure like things.

A trumpet is pretty much a trumpet. A trombone is pretty much a trombone. Even a cornet is pretty much a cornet. So comparing bore at the 2nd valve (or top slide/bottom slide on a tbone) is likely to get you a very good comparison.

Tubas aren't so easy. The American-style piston mouthpipe usually goes directly into the valve block, while the Euro-style rotary mouthpipe winds around some first. Then you have the American-style mouthpipe that has a tuning slide in it first... the point is, the valves may occur nearly anywhere in the bore profile, so getting an apples to apples comparison is REALLY difficult, even among similar valve types.

Bore size doesn't indicate anything close to what it does on the higher brass, so much so that it's really ONLY helpful comparing tubas of the same wrap of the same manufacturer.

Maker to maker is MUCH less telling. Take, for example, a piston bore vrs. a rotary bore, which occurs much later in the bore profile. Then consider my slant-rotor Marzan, which has rotary valves, but in the place usually occupied by pistons. I think it plays like a MUCH BIGGER horn than the rotary valve bore would indicate.

I never worry about bore size, just put your mouthpiece and play the damn thing, and figure it out for yourself.